A known method for consecutively producing tablets comprises feeding a metered quantity of particulate material into a die bore, compressing it to a pre-set thickness to form the tablet, measuring the final compression pressure and comparing this to lower and upper preselected pressure limits and increasing or respectively decreasing the metered quantity of particulate material fed into the die bore for subsequent tablets depending on whether the measured pressure respectively falls below the lower or exceeds the upper preselected pressure limit.
A known tablet press for use in this method comprises a die plate having a bore for receiving particulate material; an adjustable dosing device for feeding a metered amount of particulate material into the bore for forming each tablet; upper and lower rams reciprocable together in a compression stroke in the bore to compress the particulate material into a tablet and having a preset separation of the rams at the end of this compression stroke which preset separation determines the thickness of the tablet formed; means for measuring the compression pressure existing at the end of the rams' compression stroke; means for comparing said measured pressure with preselected upper and lower desired pressure limits and for generating a pressure error signal if said limits are exceeded by the measured pressure; and adjustment means responsive to said pressure error signal for varying the amount of particulate material fed to the bore by the dosing device until the measured pressure on subsequent compression strokes falls within the preselected upper and lower limits. This adjustment means may be a servo-motor controlling the dosing device.
In order to produce a plurality of tablets of approximately equal weight in a tablet press, it is thus known to monitor the pressures which occur in the press rams and in the remaining structural parts of the pressing apparatus during the pressing of the individual tablets. In this case the quantity of material which is charged into each die bore is modified by adjusting the dosing device as a function of variations in the pressure. It is assumed for this purpose that the pressure is an approximately linear function of the volume of material in the die, and hence also of the weight of the tablet pressed in the die from the material present there. If the measured pressure increases, then the quantity of material in the dies is reduced and vice versa, so that the pressure once again measures its desired value. The attempted maintenance of constant weight in the consecutively produced tablets therefore depends solely upon the maintenance of constant pressure.
The density of all the tablets produced can be monitored with such a regulating device; it is however even so possible that the tablets produced may deviate from the desired weight. The reason for this lies in the fact that, on the one hand, pressure variations are not exclusively attributable to erroneous dosings, and on the other hand that not every variation in the volume of material in the dies results in a pressure variation, because in addition to the volume of particulate material in the dies, density, moisture, grain size and grain composition or grain graduation of the material also have an influence upon the weight of the finished pressed tablet. It is therefore entirely possible for the pressure to remain constant, but for the weight of the finished tablets to vary nevertheless.
In order to maintain the tablet weight constant and to prevent the weight of any tablet deviating from the desired weight, it would be necessary to weigh each individual tablet. In the current state of the art of weighing machine construction however, it is not yet possible to construct for a reasonable outlay a weighing machine which operates even approximately as fast as a modern rotary tablet press, which rotates with a peripheral speed of over 2.5 m/sec. and turns out more than 80 tablets per second.
It is the aim of the invention to monitor the tablet weight in addition to the compression pressure and to allow for variations in the weight of the tablet in the regulation of the process and thereby to produce a regulating installation capable of maintaining the tablet weight substantially constant to a much greater extent than in the prior art.